1. Field of the Invention
The present invention relates to a current path cut-off component of a cell and more particularly to a current path cut-off device of a cell to be used for ensuring safety against an increase in internal pressure of a cell case.
2. Description of the Prior Art
For example, a rechargeable lithium ion secondary cell with an organic solvent type electrolyte poured and sealed into the interior of a cell case has heretofore been used widely as a power source of portable devices such as portable telephones and personal computers.
However, when such a cell is recharged, if the recharging is performed to an excessive degree or with an electric current larger than a predetermined current, a problem will occur in the cell, with evolution of gas, increase in internal pressure and temperature of the cell, swelling of the cell, and eventual cracking of the cell case, resulting in that the electrolyte present within the cell case oozes to the exterior and may exert adverse effect to the device in which the cell is incorporated. Even if such a problem in the cell does not lead to such a result, if the use of the cell is continued under the aforesaid occurrence of such a problem, the swelling of the cell will become more and more conspicuous, which may cause bursting of the cell. Therefore, it has been necessary to immediately stop the use of the cell upon occurrence of such a problem.
In a conventional current path cut-off component of a cell, as shown in FIGS. 8 and 9, a cell lid 3 and an upwardly displaceable actuator 4 are mounted through a gasket 2 to a case 1 of a cell which is, for example, a circular cell. The cell lid 3 is mounted by caulking for example so as to hermetically seal the interior of the cell case 1, and the actuator 4 is disposed below the cell lid 3.
Vent holes 3a are formed in the cell lid 3 so that upon displacement of the underlying actuator 4 the air present inside the cell lid can escape to the exterior through the vent holes 3a.
The actuator 4 has a disk-like safety valve portion 4a formed, for example, by drawing with use of a press. The safety valve portion 4a has a central protuberance 4b which projects downward in the figure, with radial grooves 4c being formed around the protuberance 4b.
An insulating board 5 is disposed on the underside of the actuator 4. A hole 5a for insertion therethrough of the protuberance 4a of the safety valve portion 4a and a vent hole 5b are formed through the insulating board 5.
Underlying the insulating board 5 is a lead fixing member 6, in which are formed a hole 6a communicating with the hole 5a of the insulating board 5 and a vent hole 6b communicating with the vent hole 5b of the insulating board 5.
The protuberance 4b of the safety valve portion 4a is inserted into the holes 5a and 6a of the insulating board 5 and the lead fixing member 6, respectively, and a lead 7 formed by a thin metallic plate is bonded to the tip of the protuberance 4b.
The lead 7 is bonded to the tip of the protuberance 4b, the actuator 4 and the lead 7 are in electric conduction with each other. The opposite end of the lead 7 is connected to a power generating element 8 located at a lower position in the figure, with a current path being formed between the power generating element 8 and the cell lid 3.
In the event of an increase in internal pressure of the cell case 1 due to a problem in the interior of the cell, a gas of an increased pressure flows like arrow A in FIG. 9 through the vent holes 5b and 6b and exerts an upward force on the back side of the safety vale portion 4a.
With this force, the safety valve portion 4a is raised up and the lead 7 is ruptured at its portion bonded to the protuberance 4b, whereby the current path of the cell is cut off.
As a result, the increase in internal pressure of the cell case 1 is suppressed to prevent bursting of the cell.
As portable devices become smaller in size, the recent cells using such a current path cut-off component are encountering a strong demand for the reduction of size. Such a demand is becoming more and more strong also for the conventional cells.
However, in such a conventional current path cut-off component as referred to above, unless the size of the safety valve portion 4a is made large, an increased internal pressure of the cell cannot be detected with a high accuracy. This point has been an obstacle to the reduction of size.
Moreover, in the conventional current path cut-off component of a cell, since the cell lid 3 and the actuator 4 are formed as separate members, the number of parts used becomes larger and the cell assembling operation becomes complicated.